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IN   MEMORIAM 
FLOR1AN  CAJORI 


With  the  Authors  Compliments. 


UPON     THE 


Production  of  Sound  by  Radiant  Energy, 


BY  ALEXANDER  GRAHAM   BELL. 


PAPER   READ    BEFORE    THE   NATIONAL    ACADEMY 
OF  SCIENCES,    APRIL    21,    1881. 


WASHINGTON,  D.  C. 

GIBSON   BROTHERS,  PRINTERS. 

1881. 


CAJORI 


34-3 


UPON    THE 

PRODUCTION   OF   SOUND   BY   RADIANT 
ENERGY. 

BY  ALEXANDER  GRAHAM  BELL. 


[A  Paper  read  before  the  National  Academy  of  Sciences,  April,  21,  1881.] 


IN  a  paper  read  before  the  American  Association  for  the 
Advancement  of  Science,  last  August,  I  described  certain  ex- 
periments made  by  Mr.  Sumner  Tainter  and  myself  which  had 
resulted  in  the  construction  of  a  " Phoiophone"  or  apparatus 
for  the  production  of  sound  by  light;*  and  it  will  be  my  ob- 
ject to-day  to  describe  the  progress  we  have  made  in  the  inves- 
tigation of  photophonic  phenomena  since  the  date  of  this  com- 
munication. 

In  my  Boston  paper  the  discovery  was  announced  that  thin 
disks  of  very  many  different  substances  emitted  sounds  when 
exposed  to  the  action  of  a  rapidly-interrupted  beam  of  sunlight. 
The  great  variety  of  material  used  in  these  experiments  led  me 
to  believe  that  sonorousness  under  such  circumstances  would  be 
found  to  be  a  general  property  of  all  matter. 

At  that  time  we  had  failed  to  obtain  audible  effects  from 
masses  of  the  various  substances  which  became  sonorous  in  the 
condition  of  thin  diaphragms,  but  this  failure  was  explained 
upon  the  supposition  that  the  molecular  disturbance  produced 

*  Proceedings  of  American  Association  for  the  Advancement  of  Science, 
Aug.  27th,  1880;  see,  also,  American  Journal  of  Science,  vol.  xx,  p.  305; 
Journal  of  the  American  Electrical  Society,  vol.  iii,  p.  3 ;  Journal  of  the 
Society  of  Telegraph  Engineers  and  Electricians,  vol.  ix,  p.  404 ;  Annales  de 
Chimie  et  de  Physique,  vol.  xxi. 


by  the  light  was  chiefly  a  surface  action,  and  that  under  the 
circumstances  of  the  experiments  the  vibration  had  to  be  trans- 
mitted through  the  mass  of  the  substance  in  order  to  affect  the 
ear.  It  was  therefore  supposed  that,  if  we  could  lead  to  the  ear 
air  that  was  directly  in  contact  with  the  illuminated  surface, 
louder  sounds  might  be  obtained,  and  solid  masses  be  found  to 
be  as  sonorous  as  thin  diaphragms.  The  first  experiments  made 
to  verify  this  hypothesis  pointed  towards  success.  A  beam  of 
sunlight  was  focussed  into  one  end  of  an  open  tube,  the  ear 
being  placed  at  the  other  end.  Upon  interrupting  the  beam,  a 
clear,  musical  tone  was  heard,  the  pitch  of  which  depended  upon 
the  frequency  of  the  interruption  of  the  light  and  the  loudness 
upon  the  material  composing  the  tube. 

At  this  stage  our  experiments  were  interrupted,  as  circum- 
stances called  me  to  Europe. 

While  in  Paris  a  new  form  of  the  experiment  occurred  to 
my  mind,  which  would  not  only  enable  us  to  investigate  the 
sounds  produced  by  masses,  but  would  also  permit  us  to  test 
the  more  general  proposition  that  sonorousness,  under  the  in- 
fliience  of  intermittent  HqJit,  is  a  property  common  to  all  matter. 

The  substance  to  be  tested  was  to  be  placed  in  the  interior 
of  a  transparent  vessel,  made  of  some  material  which  (like  glass) 
is  transparent  to  light,  but  practically  opaque  to  sound. 

Under  such  circumstances  the  light  could  get  in,  but  the 
sound  produced  by  the  vibration  of  the  substance  could  not  get 
out.  The  audible  effects  could  be  studied  by  placing  the  ear 
in  communication  with  the  interior  of  the  vessel  by  means  of  a 
hearing  tube. 

Some  preliminary  experiments  were  made  in  Paris  to  test 
this  idea,  and  the  results  were  so  promising  that  they  were  com- 
municated to  the  French  Academy  on  the  llth  of  October,  1880, 
in  a  note  read  for  me  by  M.  Antoine  Breguet.*  Shortly  after- 
wards I  wrote  to  Mr.  Tainter,  suggesting  that  he  should  carry 
on  the  investigation  in  America,  as  circumstances  prevented 
me  from  doing  so  myself  in  Europe.  As  these  experiments 
seem  to  have  formed  the  common  starting  point  for  a  series  of 
independent  researches  of  the  most  important  character ?  car- 
*  Comptes  Rendux,  vol.  xcl,  p.  595. 


3 

ried  on  simultaneously,  in  America  by  Mr.  Tainter,  and  in 
Europe  by  M.  Mercadier,*  Prof.  Tyndall,t  W.  E.  RontgenJ 
and  W.  II.  Preece,  §  I  may  be  permitted  to  quote  from  my 
letter  to  Mr.  Tainter  the  passage  describing  the  experiments 
referred  to : 

"  METROPOLITAN  HOTEL,  HUE  C  AMBON,  PARIS, 

"  Nov.  2,  1880. 
"  DEAR  MR.  TAINTER  : 

*  *     *     "I  have  devised  a  method  of  producing  sounds  by 
"  the  action  of  an  intermittent  beam  of  light  from  substances 
"  that  cannot  be  obtained  in  the  shape  of  thin  diaphragms  or 
"  in  the  tubular  form ;  indeed,  the  method  is  specially  adapted 
"  to  testing  the  generality  of  the  phenomenon  we  have  discov- 
"  ered,  as  it  can  be  adapted  to  solids,  liquids,  and  gases. 

"  Place  the  substance  to  be  experimented  with  in  a  glass  test- 
"  tube,  connect  a  rubber  tube  with  the  mouth  of  the  test-tube, 
"  placing  the  other  end  of  the  pipe  to  the  ear.  Then  focus 
"  the  intermittent  beam  upon  the  substance  in  the  tube.  I  have 
"  tried  a  large  number  of  substances  in  this  way  with  great 
"  success,  although  it  is  extremely  difficult  to  get  a  glimpse  of 
"  the  sun  here,  and  when  it  does  shine  the  intensity  of  the  light 
"  is  not  to  be  compared  with  that  to  be  obtained  in  Washing- 
"  ton.  I  got  splendid  effects  from  crystals  of  bichromate  of 
"  potash,  crystals  of  sulphate  of  copper,  and  from  tobacco 
"  smoke.  A  whole  cigar  placed  in  the  test-tube  produced  a 
"  very  loud  sound.  I  could  not  hear  anything  from  plain  water, 
"  but  when  the  water  was  discolored  with  ink  a  feeble  sound 
"  was  heard.  I  would  suggest  that  you  might  repeat  these  ex- 
u  periments  and  extend  the  results,"  &c.,  &c. 

Experiments  with  Solids. 

Upon  my  return  to  Washington  in  the  early  part  of  January ,|| 
Mr.  Tainter  communicated  to  me  the  results  of  the  experiments 
he  had  made  in  my  laboratory  during  my  absence  in  Europe. 

*  "  Notes  on  Radiophony,"  Comptes  Rendus,  Dec.  G  and  13,  1880;  Feb.   21 
and  28,  1881.     See,  also,  Journal  de  Physique,  vol.  x,  p.  53. 

"  Action  of  an  Intermittent  Beam  of  Radiant  Heat  upon  Gaseous  Matter." 

roc.  Royal  Society,  Jan.  13,  1881,  vol.  xxxi,  p.  307. 

I  "On  the  tones  which  arise  from  the  intermittent  illumination  of  a  gas." 
See  Annakn  der  Phyx.  und  Chemie,  Jan.,  1881,  No.  1,  p.  155. 

§  "  On  the  Conversion  of  Radiant  Energy  into  Sonorous  Vibrations."  Proc. 
Royal  Society,  March  10.  1881,  vol.  xxxi,  p.  506. 

)]  On  the  7th  of  January. 


He  had  commenced  by  examining  the  sonorous  properties  of 
a  vast  number  of  substances  enclosed  in  test-tubes  in  a  simple 
empirical  search  for  loud  effects.  He  was  thus  led  gradually 
to  the  discovery  that  cotton-wool,  worsted,  silk,  and  fibrous 
materials  generally,  produced  much  louder  sounds  than  hard 
rigid  bodies  like  crystals,  or  diaphragms  such  as  we  had  hitherto 
used. 

In  order  to  study  the  effects  under  better  circumstances  he 
enclosed  his  materials  in  a  conical  cavity  in  a  piece  of  brass 
closed  by  a  flat  plate  of  glass.  A  brass  tube  leading  into  the 
cavity  served  for  connection  with  the  hearing-tube.  When 
this  conical  cavity  was  stuffed  with  worsted  or  other  fibrous 
materials  the  sounds  produced  were  much  louder  than  when 
a  test- tube  was  employed.  This  form  of  receiver  is  shown  in 
Figure  I. 

Mr.  Tainter  next  collected  silks  and  worsteds  of  different 
colors,  and  speedily  found  that  the  darkest  shades  produced  the 
best  effects.  Black  worsted  especially  gave  an  extremely  loud 

sound. 

As  white  cotton- wool  had  proved  itself  equal,  if  not  superior, 
to  any  other  white  fibrous  material  before  tried,  he  was  anxious 
to  obtain  colored  specimens  for  comparison.  Not  having  any 
at  hand,  however,  he  tried  the  effect  of  darkening  some  cotton- 
wool with  lamp-black.  Such  a  marked  reinforcement  of  the 
sound  resulted  that  he  was  induced  to  try  lamp-black  alone. 

About  a  teaspoonf  ul  of  lamp-black  was  placed  in  a  test-tube 
and  exposed  to  an  intermittent  beam  of  sunlight.  The  sound 
produced  was  much  louder  than  any  heard  before. 

Upon  smoking  a  piece  of  plate-glass,  and  holding  it  in  the 
intermittent  beam  with  the  lamp-black  surface  towards  the  sun, 
the  sound  produced  was  loud  enough  to  be  heard,  with  atten- 
tion, in  any  part  of  the  room.  With  the  lamp-black  surface 
turned  from  the  sun  the  sound  was  much  feebler. 

Mr.  Tainter  repeated  these  experiments  for  me  immediately 
upon  my  return  to  Washington,  so  that  I  might  verify  his 
results. 

Upon  smoking  the  interior  of  the  conical  cavity  shown  in 
Figure  I,  and  then  exposing  it  to  the  intermittent  beam,  with 


the  glass  lid  in  position  as  shown,  the  effect  was  perfectly 
startling.  The  sound  was  so  loud  as  to  be  actually  painful  to 
an  ear  placed  closely  against  the  end  of  the  hearing-tube. 

The  sounds,  however,  were  sensibly  louder  when  we  placed 
some  smoked  wire  gauze  in  the  receiver,  as  illustrated  in  the 
drawing,  Figure  1. 

When  the  beam  was  thrown  into  a  resonator,  the  interior  of 
which  had  been  smoked  over  a  lamp,  most  curious  alternations 
of  sound  and  silence  were  observed.  The  interrupting  disk 
was  set  rotating  at  a  high  rate  of  speed,  and  was  then  allowed 
to  come  gradually  to  rest.  An  extremely  feeble  musical  tone 
was  at  first  heard,  which  fell  in  pitch  as  the  rate  of  interrup- 
tion grew  less.  The  loudness  of  the  sound  produced  varied  in 
the  most  interesting  manner.  Minor  reinforcements  were  con- 
stantly occurring,  which  became  more  and  more  marked  as  the 
true  pitch  of  the  resonator  was  neared.  When  at  last  the  fre- 
quency of  interruption  corresponded  to  the  frequency  of  the 
fundamental  of  the  resonator,  the  sound  was  so  loud  that  it 
might  have  been  heard  by  an  audience  of  hundreds  of  people. 

The  effects  produced  by  lamp-black  seemed  to  me  to  be  very 
extraordinary,  especially  as  I  had  a  distinct  recollection  of  ex- 
periments made  in  the  summer  of  1880  with  smoked  diaphragms, 
in  which  no  such  reinforcement  was  noticed. 

Upon  examining  the  records  of  our  past  photophonic  experi- 
ments we  found  in  vol.  vii,  p.  57,  the  following  note: 

"  Experiment  Y. — Mica  diaphragm  covered  with  lamp-black 
on  side  exposed  to  light. 

"Result :  distinct  sound  about  same  as  without  lampblack. — 
A.  G.  B.,  July  18^,  1880. 

"Verified  the  above,  but  think  it  somewhat  louder  than 
when  used  without  lamp-black."— #.  T.,  July  ISth,  1880. 

Upon  repeating  this  old  experiment  we  arrived  at  the  same 
result  as  that  noted.  Little  if  any  augmentation  of  sound  re- 
sulted from  smoking  the  mica.  In  this  experiment  the  effect 
was  observed  by  placing  the  mica  diaphragm  against  the  ear  arid 
also  by  listening  through  a  hearing-tube,  one  end  of  which  was 
closed  by  the  diaphragm.  The  sound  was  found  to  be  more 


6 

audible  through  the  free  air  when  the  ear  was  placed  as  near 
to  the  lamp-black  surface  as  it  could  be  brought  without 
shading  it.  Thus  the  vibrations  produced  in  lamp-black  under 
the  above  circumstances  do  not  appear  to  be  communicated  to 
any  very  appreciable  extent  to  the  diaphragm  on  which  the 
lamp-black  is  deposited. 

At  the  time  of  my  communication  to  the  American  Associa- 
tion I  had  been  unable  to  satisfy  myself  that  the  substances 
which  had  become  sonorous  under  the  direct  influence  of  inter- 
mittent sunlight  were  capable  of  reproducing  the  sounds  of 
articulate  speech  under  the  action  of  an  undulatory  beam  from 
our  photophonic  transmitter.  The  difficulty  in  ascertaining 
this  will  be  understood  by  considering  that  the  sounds  emitted 
by  thin  diaphragms  and  tubes  were  so  feeble  that  it  was  im- 
practicable to  produce  audible  effects  from  substances  in  these 
conditions  at  any  considerable  distance  from  the  transmitter; 
but  it  was  equally  impossible  to  judge  of  the  effects  produced 
by  our  articulate  transmitter  at  a  short  distance  away  because 
the  speaker's  voice  was  directly  audible  through  the  air.  The 
extremely  loud  sounds  produced  from  lamp-black  have  enabled 
us  to  demonstrate  the  feasibility  of  using  this  substance  in  an 
articulating  photophone  in  place  of  the  electrical  receiver  for- 
merly employed. 

The  drawing  (Fig.  2)  illustrates  the  mode  in  which  the  experi- 
ment was  conducted.  The  diaphragm  of  the  transmitter  (A) 
was  only  5  centimetres  in  diameter,  the  diameter  of  the  re- 
ceiver (B)  was  also  5  centimetres,  and  the  distance  between  the 
two  was  40  metres,  or  800  times  the  diameter  of  the  transmit- 
ting diaphragm.  We  were  unable  to  experiment  at  greater 
distances  without  a  heliostat  on  account  of  the  difficulty  of 
keeping  the  light  steadily  directed  on  the  receiver.  Words 
and  sentences  spoken  into  the  transmitter  in  a  low  tone  of  voice 
were  audibly  reproduced  by  the  lamp-black  receiver. 

In  Fig.  3  is  shown  a  mode  of  interrupting  a  beam  of  sunlight 
for  producing  distant  effects  without  the  use  of  lenses.  Two 
similarly-perforated  disks  are  employed,  one  of  which  is  set  in 
rapid  rotation,  while  the  other  remains  stationary.  This  form 
of  interrupter  is  also  admirably  adapted  for  work  with  artificial 


11 

light.  The  receiver  illustrated  in  the  drawing  consists  of  a 
parabolic  reflector,  in  the  focus  of  which  is  placed  a  glass  ves- 
sel (A)  containing  lamp-black  or  other  sensitive  substance,  and 
connected  with  a  hearing-tube.  The  beam  of  light  is  inter- 
rupted by  its  passage  through  the  two  slotted  disks  shown  at  B, 
and  in  operating  the  instrument  musical  signals  like  the  dots 
and  dashes  of  the  Morse  alphabet  are  produced  from  the  sensi- 
tive receiver  (A)  by  slight  motions  of  the  mirror  (C)  about  its 
axis  (D.) 

In  place  of  the  parabolic  reflector  shown  in  the  figure  a  coni- 
cal reflector  like  that  recommended  by  Prof.  Sylvanus  Thomp- 
son* can  be  used,  in  which  case  a  cylindrical  glass  vessel  would 
be  preferable  to  the  flask  (A)  shown  in  the  figure. 

In  regard  to  the  sensitive  materials  that  can  be  employed, 
our  experiments  indicate  that  in  the  case  of  solids  the  physical 
condition  and  the  color  markedly  influence  the  intensity  of 
the  sonorous  effects.  The  loudest  sounds  are  produced  from 
substances  in  a  loose,  porous,  spongy  condition,  and  from  those 
that  have  the  darkest  or  most  absorbent  colors. 

The  materials  from  which  the  best  effects  have  been  obtained 
are  cotton-wool,  worsted,  fibrous  materials  generally,  cork, 
sponge,  platinum  and  other  metals  in  a  spongy  condition,  and 
lamp-black. 

The  loud  sounds  produced  from  such  substances  may  per- 
haps be  explained  in  the  following  mariner:  Let  us  consider, 
for  example,  the  case  of  lamp-black — a  substance  which  be- 
comes heated  by  exposure  to  rays  of  all  refrangibility.  I  look 
upon  a  mass  of  this  substance  as  a  sort  of  sponge,  with  its  pores 
filled  with  air  instead  of  water.  When  a  beam  of  sunlight 
falls  upon  this  mass,  the  particles  of  lamp-black  are  heated,  and 
consequently  expand,  causing  a  contraction  of  the  air-spaces 
or  pores  among  them. 

Under  these  circumstances  a  pulse  of  air  should  be  expelled, 
just  as  we  would  squeeze  out  water  from  a  sponge. 

The  force  with  which  the  air  is  expelled  must  be  greatly  in- 
creased by  the  expansion  of  the  air  itself,  due  to  contact  with 
the  heated  particles  of  lamp-black.  When  the  light  is  cut  off 

*Pbil.  Mag.,  April,  1881,  vol.  xi,  p.  286. 


12 

the  converse  process  takes  place.  The  lamp-black  particles 
cool  and  contract,  thus  enlarging  the  air  spaces  among  them, 
and  the  enclosed  air  also  becomes  cool.  Under  these  circum- 
stances a  partial  vacuum  should  be  formed  among  the  particles, 
and  the  outside  air  would  then  be  absorbed,  as  water  is  by  a 
sponge  when  the  pressure  of  the  hand  is  removed. 

I  imagine  that  in  some  such  manner  as  this  a  wave  of  con- 
densation is  started  in  the  atmosphere  each  time  a  beam  of  sun- 
light falls  upon  lamp-black,  and  a  wave  of  rarefaction  is  origi- 
nated when  the  light  is  cut  off.  We  can  thus  understand 
how  it  is  that  a  substance  like  lamp-black  produces  intense 
sonorous  vibrations  in  the  surrounding  air,  while  at  the  same 
time  it  communicates  a  very  feeble  vibration  to  the  diaphragm 
or  solid  bed  'upon  which  it  rests. 

This  curious  fact  was  independently  observed  in  England  by 
Mr.  Preece,  and  it  led  him  to  question  whether,  in  our  experi- 
ments with  thin  diaphragms,  the  sound  heard  was  due  to  the 
vibration  of  the  disk  or  (as  Prof.  Hughes  had  suggested)  to  the 
expansion  and  contraction  of  the  air  in  contact  with  the  disk 
confined  in  the  cavity  behind  the  diaphragm.  In  his  paper  read 
before  the  Royal  Society  on  the  10th  of  March,  Mr.  Preece  de- 
scribes experiments  from  which  he  claims  to  have  proved  that 
the  effects  are  wholly  due  to  the  vibrations  of  the  confined  air, 
and  that  the  disks  do  not  vibrate  at  all. 

I  shall  briefly  state  my  reasons  for  disagreeing  with  him  in 
this  conclusion : 

1.  When  an  intermittent  beam  of  sunlight  is  focussed  upon 
a  sheet  of  hard  rubber  or  other  material,  a  musical  tone  can  be 
heard,  not  only  by  placing  the  ear  immediately  behind  the  part 
receiving  the  beam,  but  by  placing  it  against  any  portion  of  the 
sheet,  even  though  this  may  be  a  foot  or  more  from  the  place 
acted  upon  by  the  light. 

2.  When  the  beam  is  thrown  upon  the  diaphragm  of  a  "  Blake 
Transmitter,"  a  loud  musical  tone  is  produced  by  a  telephone 
connected  in  the  same  galvanic  circuit  with  the  carbon  button, 
(A,)  Fig.  4.     Good  effects  are  also  produced  when  the  carbon 
button  (A)  forms,  with  the  battery,  (B,)  a  portion  of  the  pri- 
mary  circuit  of  an  induction  coil,  the  telephone   (C)  being 
placed  in  the  secondary  circuit. 


Fig.  6. 


Fig.  5. 


n 

In  these  cases  the  wooden  box  and  mouth-piece  of  the  trans- 
mitter should  be  removed,  so  that  no  air-cavities  may  be  left  on 
either  side  of  the  diaphram. 

It  is  evident,  therefore,  that  in  the  case  of  thin  disks  a  real 
vibration  of  the  diaphragm  is  caused  by  the  action  of  the  in- 
termittent beam,  independently  of  any  expansion  and  contrac- 
tion of  the  air  confined  in  the  cavity  behind  the  dw/phrd^m. 

Lord  Rayleigh  has  shown  mathematically  that  a  to-and-fro 
vibration,  of  sufficient  amplitude  to  produce  an  audible  sound, 
would  result  from  a  periodical  communication  and  abstraction 
of  heat,  and  he  says:  "We  may  conclude,  I  think,  that  there 
"  is  at  present  no  reason  for  discarding  the  obvious  explanation 
"  that  the  sounds  in  question  are  due  to  the  bending  of  the 
"  plates  under  unequal  heating."  (Nature,  xxiii,  p.  274.) 
Mr.  Preece,  however,  seeks  to  prove  that  the  sonorous  effects 
cannot  be  explained  upon  this  supposition ;  but  his  experimental 
data  are  not  sufficient  to  support  his  conclusion.  Mr.  Preece 
expected  that  if  Lord  Rayleigh's  explanation  was  correct,  the 
expansion  and  contraction  of  a  thin  strip  under  the  influence 
of  an  intermittent  beam  could  be  caused  to  open  and  close  a 
galvanic  circuit  so  as  to  produce  a  musical  tone  from  a  tele- 
phone in  the  circuit.  But  this  was  an  inadequate  way  to  test 
the  point  at  issue,  for  Lord  Rayleigh  has  shown  (Proc.  of  Roy. 
Soc.,  1877)  that  an  audible  sound  can  be  produced  by  a  vibra- 
tion whose  amplitude  is  less  than  a  ten-millionth  of  a  centime- 
tre, and  certainly  such  a  vibration  as  that  would  not  have  suf- 
ficed to  operate  a  "  make-and-break  contact"  like  that  used  by 
Mr.  Preece.  The  negative  results  obtained  by  him  cannot, 
therefore,  be  considered  conclusive. 

The  following  experiments  (devised  by  Mr.  Tainter)  have 
given  results  decidedly  more  favorable  to  the  theory  of  Lord 
Rayleigh  than  to  that  of  Mr.  Preece : 

1.  A  strip  (A)  similar  to  that  used  in  Mr.  Preece's  experi- 
ment was  attached  firmly  to  the  centre  of  an  iron  diaphragm, 
(B,)  as  shown  in  Figure  5,  and  was  then  pulled  taut  at  right 
angles  to  the  plane  of  the  diaphragm.  When  the  intermittent 
beam  was  focussed  upon  the  strip  (A)  a  clear  musical  tone 
could  be  heard  by  applying  the  ear  to  the  hearing-tube  (C.) 


18 

This  seemed  to  indicate  a  rapid  expansion  and  contraction  of 
the  substance  under  trial. 

But  a,  vibration  of  the  diaphragm  (B)  would  also  have  re- 
sulted if  the  thin  strip  (A)  had  acquired  a  to-and-fro  motion, 
due  either  to  the  direct  impact  of  the  beam  or  to  the  sudden 
expansion  of  the  air  in  contact  with  the  strip. 

2.  To  test  whether  this  had  been  the  case  an  additional  strip 
(D)  was  attached  by  its  central  point  only  to  the  strip  under 
trial,  and  was  then  submitted  to  the  action  of  the  beam,  as 
shown  in  Fig.  6. 

It  was  presumed  that  if  the  vibration  of  the  diaphragm  (B) 
had  been  due  to  a  pushing  force  acting  on  the  strip  (A,) 
that  the  addition  of  the  strip  (D)  would  not  interfere  with 
the  effect.  But  if,  on  the  other  hand,  it  had  been  due  to  the 
longitudinal  expansion  and  contraction  of  the  strip  (A,)  the 
sound  would  cease,  or  at  least  be  reduced.  The  beam  of  light 
falling  upon  strip  (D)  was  now  interrupted  as  before  by  the 
rapid  rotation  of  a  perforated  disk,  which  was  allowed  to  come 
gradually  to  rest. 

No  sound  was  heard  excepting  at  a  certain  speed  of  rotation, 
when  a  feeble  musical  tone  became  audible. 

This  result  is  confirmatory  of  the  first. 

The  audibility  of  the  effect  at  a  particular  rate  of  interruption 
suggests/ the  explanation  that  the  strip  D  had  a  normal  rate 
of  vibration  of  its  own. 

When  the  frequency  of  the  interruption  of  the  light  corres- 
ponded to  this,  the  strip  was  probably  thrown  into  vibration 
after  the  manner  of  a  tuning-fork,  in  which  case  a  to-and-fro 
vibration  would  be  propagated  down  its  stem  or  central  support 
to  the  strip  (A.) 

This  indirectly  proves  the  value  of  the  experiment. 

The  list  of  solid  substances  that  have  been  submitted  to 
experiment  in  my  laboratory  is  too  long  to  be  quoted  here,  and 
I  shall  merely  say  that  we  have  not  yet  found  one  solid  body 
that  has  failed  to  become  sonorous  under  proper  conditions  of 
experiment.* 

Experiments  with  Liquids. 
The  sounds  produced  by  liquids  are  much  more  difficult  to 

*  Carbon  and  thin  microscope  glass  are  mentioned  in  my  Boston  paper  as 
non-responsive,  and  powdered  chlorate  of  potash  in  the  communication  to  the 
French  Academy,  (Comptes  Rendus,  vol.  xcl,  p.  595.)  All  these  substances 
have  since  yielded  sounds  under  more  careful  conditions  of  experiment. 


19 

observe  than  those  produced  by  solids.  The  high  absorptive 
power  possessed  by  most  liquids  would  lead  one  to  expect  in- 
tense vibrations  from  the  action  of  intermittent  light,  but  the 
number  of  sonorous  liquids  that  have  so  far  been  found  is  ex- 
tremely limited,  and  the  sounds  produced  are  so  feeble  as  to 
be  heard  only  by  the  greatest  attention  and  under  the  best  cir- 
cumstances of  experiment.  In  the  experiments  made  in  my 
laboratory  a  very  long  test-tube  was  filled  with  the  liquid  under 
examination,  and  a  flexible  rubber-tube  was  slipped  over  the 
mouth  far  enough  down  to  prevent  the  possibility  of  any  light 
reaching  the  vapor  above  the  surface.  Precautions  were  also 
taken  to  prevent  reflection  from  the  bottom  of  the  test-tube. 
An  intermittent  beam  of  sunlight  was  then  focussed  upon  the 
liquid  in  the  middle  portion  of  the  test-tube  by  means  of  a  lens 
of  large  diameter. 

Results. 

Clear  water No  sound  audible- 
Water  discolored  by  ink Feeble  sound. 

Mercury No  sound  heard. 

Sulphuric  ether* Feeble,  but  distinct  sound. 

Ammonia "         "         "  " 

Ammonio-sulphate  of  copper 

Writing  ink 

Indigo  in  sulphuric  acid 

Chloride  of  copper  * 


tt  u  a  .. 


The  liquids  distinguished  by  an  asterisk  gave  the  best 
sounds. 

Acoustic  vibrations  are  always  much  enfeebled  in  passing 
from  liquids  to  gases,  and  it  is  probable  that  a  form  of  experi- 
ment may  be  devised  which  .will  yield  better  results  by  com- 
municating the  vibrations  of  the  liquid  to  the  ear  through  the 
medium  of  a  solid  rod. 

Experiments  with  Gaseous  Matter. 

On  the  29th '  of  November,  1880,  I  had  the  pleasure  of 
showing  to  Prof.  Tyndall  in  the  laboratory  of  the  Royal  Insti- 
tution the  experiments  described  in  the  letter  to  Mr.  Tainter 
from  which  I  have  quoted  above,  and  Prof.  Tyndall  at  once 


20 

expressed  the  opinion  that  the  sounds  were  due  to  rapid 
changes  of  temperature  in  the  body  submitted  to  the  action  of 
the  beam.  Finding  that  no  experiments  had  been  made  at  that 
time  to  test  the  sonorous  properties  of  different  gases,  he  sug- 
gested filling  one  tes£4ube  with  the  vapor  of  sulphuric  ether, 
(aTgood  absorbent  of  heat,)  and  another  with  the  vapor  of  bi- 
sulphide of  carbon,  (a  poor  absorbent,)  and  he  predicted  that  if 
any  sound  was  heard  it  would  be  louder  in  the  former  case  than 
in  the  latter. 

The  experiment  was  immediately  made,  and  the  result  veri- 
fied the  prediction. 

Since  the  publication  of  the  memoirs  of  Rontgen  *  and  Tyn- 
dall  f  we  have  repeated  these  experiments,  and  have  extended 
the  inquiry  to  a  number  of  other  gaseous  bodies,  obtaining  in 
every  case  similar  results  to  those  noted  in  the  memoirs  re- 
ferred to. 

The  vapors  of  the  following  substances  w^ere  found  to  be 
highly  sonorous  in  the  intermittent  beam:  Water  vapor,  coal 
gas,  sulphuric  ether,  alcohol,  ammonia,  amylene,  ethyl  bromide, 
diethylamene,  mercury,  iodine,  and  peroxide  of  nitrogen.  The 
loudest  sounds  were  obtained  from  iodine  and  peroxide  of 
nitrogen. 

I  have  now  shown  that  sounds  are  produced  by  the  direct 
action  of  intermittent  sunlight  from  substances  in  every  physi- 
cal condition,  (solid,  liquid,  and  gaseous,)  and  the  probability 
is  therefore  very  greatly  increased  that  sonorousness  under  such 
circumstances  will  be  found  to  be  a  universal  property  of 
matter. 

Upon  Substitutes  for  Selenium  in  Electrical  Receivers. 

At  the  time  of  my  communication  to  the  American  Associa- 
tion the  loudest  effects  obtained  were  produced  by  the  use  of 
selenium,  arranged  in  a  cell  of  suitable  construction,  and  placed 
in  a  galvanic  circuit  with  a  telephone.  Upon  allowing  an  in- 
termittent beam  of  sunlight  to  fall  upon  the  selenium  a  musical 

*  Ann.  der  Phys.  und  Chem.,  1881,  No.  1,  p.  155. 
tProc.  Roy.  Soc.,  vol.  xxxi,  p.  307. 


Pigr-  7. 


tone  of  great  intensity  was  produced  from  the  telephone  con- 
nected with  it. 

But  the  selenium  was  very  inconstant  in  its  action.  Two 
pieces  of  selenium  (even  of  the  same  stick)  seldom  yielded  the 
same  results  under  identical  circumstances  of  annealing,  &c. 
While  in  Europe  last  autumn,  Dr.  Chichester  Bell,  of  Univer- 
sity College,  London,  suggested  to  me  that  this  inconstancy  of 
result  might  be  due  to  chemical  impurities  in  the  selenium  used. 
Dr.  Bell  has  since  visited  my  laboratory  in  Washington,  and 
has  made  a  chemical  examination  of  the  various  samples  of  se- 
lenium I  had  collected  from  different  parts  of  the  world.  As 
I  understand  it  to  be  his  intention  to  publish  the  results  of  this 
analysis  very  soon,  I  shall  make  no  further  mention  of  his  in- 
vestigation than  to  state  that  lie  has  found  sulphur,  iron,  lead, 
and  arsenic  in  the  so-called  "  selenium,"  with  traces  of  organic 
matter  ;  that  a  quantitative  examination  has  revealed  the  fact 
that  sulphur  constitutes  nearly  one  per  cent,  of  the  whole  mass ; 
and  that  when  these  impurities  are  eliminated  the  selenium  ap- 
pears to  be  more  constant  in  its  action  and  more  sensitive  to 
light. 

Prof.  W.  G.  Adams*  has  shown  that  tellurium,  like  selenium, 
has  its  electrical  resistance  affected  by  light,  and  we  have  at- 
tempted to  utilize  this  substance  in  place  of  selenium.  The 
arrangement  of  cell  (shown  in  Fig.  7)  was  constructed  for  this 
purpose  in  the  early  part  of  1880 ;  but  we  failed  at  that  time 
to  obtain  any  indications  of  sensitiveness  with  a  reflecting  gal- 
vanometer. We  have  since  found,  however,  that  when  this 
tellurium  spiral  is  connected  in  circuit  with  a  galvanic  battery 
and  telephone,  and  exposed  to  the  action  of  an  intermittent 
beam  of  sunlight,  a  distinct  musical  tone  is  produced  by  the  tele- 
phone. The  audible  effect  is  much  increased  by  placing  the 
tellurium  cell  with  the  battery  in  the  primary  circuit  of  an  in- 
duction coil,  and  placing  the  telephone  in  the  secondary  circuit. 

The  enormously  high  resistance  of  selenium  and  the  ex- 
tremely low  resistance  of  tellurium  suggested  the  thought  that 
an  alloy  of  these  two  substances  might  possess  intermediate 
electrical  properties.  We  have  accordingly  mixed  together 

*Proc.  Roy.  Soc.,  vol.  xxiv,  p.  163. 


selenium  and  tellurium  in  different  proportions,  and  while  we 
do  not  feel  warranted  at  the  present  time  in  making  definite 
statements  concerning  the  results,  I  may  say  that  such  alloys 
have  proved  to  be  sensitive  to  the  action  of  light. 

It  occurred  to  Mr.  Tainter  before  my  return  to  Washington 
last  January  that  the  very  great  molecular  disturbance  pro- 
duced in  lamp-black  by  the  action  of  intermittent  sunlight 
should  produce  a  corresponding  disturbance  in  an  electrical  cur- 
rent passed  through  it,  in  which  case  lamp-black  could  be  em- 
ployed in  place  of  selenium  in  an  electrical  receiver.  This  has 
turned  out  to  be  the  case,  and  the  importance  of  the  discovery 
is  very  great,  especially  when  we  consider  the  expense  of  such 
rare  substances  as  selenium  and  tellurium. 

The  form  of  lamp-black  cell  we  have  found  most  effective  is 
shown  in  Fig.  8.  Silver  is  deposited  upon  a  plate  of  glass, 
and  a  zigzag  line  is  then  scratched  through  the  lilm,  as  shown, 
dividing  the  silver  surface  into  two  portions  insulated  from  one 
another,  having  the  form  of  two  combs  with  interlocking  teeth. 

Each  comb  is  attached  to  a  screw-cup,  so  that  the  cell  can  be 
placed  in  an  electrical  circuit  when  required.  The  surface  is 
then  smoked  until  a  good  film  of  lamp-black  is  obtained,  filling 
the  interstices  between  the  teeth  of  the  silver  combs.  When 
the  lamp-black  cell  is  connected  with  a  telephone  and  galvanic 
battery,  and  exposed  to  the  influence  of  an  intermittent  beam 
of  sunlight,  a  loud  musical  tone  is  produced  by  the  telephone. 
This  result  seems  to  be  due  rather  to  the  physical  condition 
than  to  the  nature  of  the  conducting  material  employed,  as 
metals  in  a  spongy  condition  produce  similar  effects.  For  in- 
stance, when  an  electrical  current  is  passed  through  spongy 
platinum  while  it  is  exposed  to  intermittent  sunlight,  a  dis- 
tinct musical  tone  is  produced  by  a  telephone  in  the  same  cir- 
cuit. In  all  such  cases  the  effect  is  increased  by  the  use  of  an 
induction  coil;  and  the  sensitive  cells  can  be  employed  for  the 
reproduction  of  articulate  speech  as  well  as  for  the  production 
of  musical  sounds. 

We  have  also  found  that  loud  sounds  are  produced  from 
lamp-black  by  passing  through  it  an  intermittent  electrical 


27 

current ;  and  that  it  can  be  used  as  a  telephonic  receiver  for 
the  reproduction  of  articulate  speech  by  electrical  means. 

A  convenient  mode  of  arranging  a  lamp-black  cell  for  ex- 
perimental purposes  is  shown  in  Fig.  9.  When  an  intermittent 
current  is  passed  through  the  lamp-black,  (A,)  or  when  an  in- 
termittent beam  of  sunlight  falls  upon  it  through  the  glass 
plate  B,  a  loud  musical  tone  can  be  heard  by  applying  the  ear 
to  the  hearing-tube  C.  When  the  light  and  the  electrical  cur- 
rent act  simultaneously,  two  musical  tones  are  perceived,  which 
produce  beats  when  nearly  of  the  same  pitch.  By  proper  ar- 
rangements a  complete  interference  of  sound  can  undoubtedly 
be  produced. 

Upon  the  Measurement  of  the  Sonorous  Effects  Produced  by 
Different  Substances. 

We  have  observed  that  different  substances  produce  sounds 
of  very  different  intensities  under  similar  circumstances  of  ex- 
periment, and  it  has  appeared  to  us  that  very  valuable  informa- 
tion might  be  obtained  if  we  could  measure  the  audible  effects 
produced.  For  this  purpose  wre  have  constructed  several  differ- 
ent forms  of  apparatus  for  studying  the  effects,  but  as  our  re- 
searches are  not  yet  complete,  I  shall  confine  myself  to  a  sim- 
ple description  of  some  of  the  forms  of  apparatus  we  have  de- 
vised. 

When  a  beam  of  light  is  brought  to  a  focus  by  means  of  a 
lens,  the  beam  diverging  from  the  focal  point  becomes  weaker 
as  the  distance  increases  in  a  calculable  degree.  Hence,  if 
we  can  determine  the  distances  from  the  focal  point  at  which 
two  different  substances  emit  sounds  of  equal  intensity,  we  can 
calculate  their  relative  sonorous  powers. 

Preliminary  experiments  were  made  by  Mr.  Tainter  during 
my  absence  in  Europe  to  ascertain  the  distance  from  the  focal 
point  of  a  lens  at  which  the  sound  produced  by  a  substance 
became  inaudible.  A  few  of  the  results  obtained  will  show 
the  enormous  differences  existing  between  different  substances 
in  this  respect. 


28 

Distance  from  Focal  Point  of  Lens  at  which  Sounds  became 
Inaudible  with  Different  Substances. 

Zinc  diaphragm,  (polished) 1.51  m 

Hard  rubber  diaphragm 1.90  " 

Tin-foil  "  2.00  " 

Telephone  "  (Japanned  iron) 2.15  " 

Zinc  "  (unpolished) 2.15  " 

White  silk,  (In  receiver  shown  in  Fig.  1.).  ...  3.10  " 

White  worsted,  "  "  "  "  .  .  -  -  4.01  " 

Yellow  worsted,  "  "  "  «  ....  4.06  « 

Yellow  silk,  "  "  "  u  ....  4.13  " 

White  cotton-wool,  "  "  "  "  4.38  " 

Green  silk,  "  "  "  "  ....  4.52  " 

Blue  worsted,  "  "  "  "  . .  . .  4.69  " 

Purple  silk,  "  "  "  «  ....  4.82  " 

Brown  silk,  "  "  "  «  ....  5.02  " 

Black  silk,  "  "  "  "  ....  5.21  " 

Red  silk,  "  "  "  «  ..'..  5.24  « 

Black  worsted,  "  "  ....  6.50  " 
Lamp-black.  In  this  case  the  limit  of  audibility  could 

not  be  determined  on  account  of  want  of  space. 

Sound  perfectly  audible  at  a  distance  of 10.00  " 

Mr.  Tainter  was  convinced  from  these  experiments  that  this 
field  of  reseach  promised  valuable  results,  and  he  at  once  de- 
vised an  apparatus  for  studying  the  effects,  which  he  described 
to  me  upon  my  return  from  Europe.  The  apparatus  has  since 
been  constructed  and  I  take  great  pleasure  in  showing  it  to  you 
to-day. 

(1.)  A  beam  of  light  is  received  by  two  similar  lenses,  (A  B, 
Fig.  10,)  which  bring  the  light  to  a  focus  on  either  side  of 
the  interrupting  disk  (C.)  The  two  substances,  whose  sonorous 
powers  are  to  be  compared,  are  placed  in  the  receiving  vessels 
(D  E)  (so  arranged  as  to  expose  equal  surfaces  to  the  action 
of  the  beam)  which  communicate  by  flexible  tubes  (F  G)  of 
equal  length,  with  the  common  hearing-tube  (H.)  The  re- 
ceivers (D  E)  are  placed  upon  slides,  which  can  be  moved 
along  the  graduated  supports  (I  K.)  The  beams  of  light  pass- 
ing through  the  interrupting  disk  (C)  are  alternately  cut  off  by 
the  swinging  of  a  pendulum,  (L.)  Thus  a  musical  tone  is 
produced  alternately  from  the  substance  in  D  and  from  that 


31 

in  E.  One  of  the  receivers  is  kept  at  a  constant  point  upon 
its  scale,  and  the  other  receiver  is  moved  towards  or  from  the 
focus  of  its  beam  until  the  ear  decides  that  the  sounds  pro- 
duced from  1)  and  E  are  of  equal  intensity.  The  relative  po- 
sitions of  the  receivers  are  then  noted. 

(2.)  Another  method  of  investigation  is  based  upon  the  pro- 
duction of  an  interference  of  sound,  and  the  apparatus  employed 
is  shown  in  Fig.  11.  The  interrupter  consists  of  a  tuning- 
fork,  (A,)  which  is  kept  in  continuous  vibration  by  means  of 
an  electro-magnet,  (B.) 

A  powerful  beam  of  light  is  brought  to  a  focus  between  the 
prongs  of  the  tuning-fork,  (A,)  and  the  passage  of  the  beam  is 
more  or  less  obstructed  by  the  vibration  of  the  opaque  screens 
(C  D)  carried  by  the  prongs  of  the  fork. 

As  the  tuning-fork  (A)  produces  a  sound  by  its  own  vibra- 
tion, it  is  placed  at  a  sufficient  distance  away  to  be  inaudible 
through  the  air,  and  a  system  of  lenses  is  employed  for  the  pur- 
pose of  bringing  the  undulating  beam  of  light  to  the  receiving 
lens  (E)  with  as  little  loss  as  possible.  The  two  receivers  (F 
G)  are  attached  to  slides  which  move  upon  the  graduated  sup- 
ports (H  I)  on  opposite  sides  of  the  axis  of  the  beam,  and  the 
receivers  are  connected  by  flexible  tubes  of  unequal  length  (K 
L)  communicating  with  the  common  hearing-tube  (M.) 

The  length  of  the  tube  (K)  is  such  that  the  sonorous  vibra- 
tions from  the  receivers  (F  G)  reach  the  common  hearing-tube 
(M)  in  opposite  phases.  Under  these  circumstances  silence  is 
produced  when  the  vibrations  in  the  receivers  (F  G)  are  of 
equal  intensity.  When  the  intensities  are  unequal,  a  residual 
effect  is  perceived.  In  operating  the  instrument  the  position 
of  the  receiver  (G)  remains  constant,  and  the  receiver  (F)  is 
moved  to  or  from  the  focus  of  the  beam  until  complete  silence 
is  produced.  The  relative  positions  of  the  two  receivers  are 
then  noted. 

(3.)  Another  mode  is  as  follows:  The  loudness  of  a  musical 
tone  produced  by  the  action  of  light  is  compared  with  the 
loudness  of  a  tone  of  similar  pitch  produced  by  electrical 
means.  A  rheostat  introduced  into  the  circuit  enables  us  to 


32 

measure  the  amount  of  resistance  required  to  render  the  elec- 
trical sound  equal  in  intensity  to  the  other. 

(4.)  If  the  tuning-fork  (A)  in  Fig.  11  is  thrown  into  vibra- 
tion by  an  midulatory  instead  of  an  intermittent  current  passed 
through  the  electro-magnet,  (B,)  it  is  probable  that  a  musical 
tone,  electrically  produced  in  the  receiver  (F)  by  the  action  of 
the  same  current,  would  be  found  capable  of  extinguishing  the 
effect  produced  in  the  receiver  (G)  by  the  action  of  the  undu- 
latory  beam  of  light,  in  which  case  it  should  be  possible  to 
establish  an  acoustic  balance  between  the  effects  produced  by 
light  and  electricity  by  introducing  sufficient  resistance  into  the 
electric  circuit.. 

Upon  the  Nature  of  the  Rays  that  Produce  Sonorous  Effects 
in  Different  Substances. 

In  my  paper  read  before  the  American  Association  last 
August  and  in  the  present  paper  I  have  used  the  word  "light" 
in  its  usual  rather  than  its  scientific  sense,  and  I  have  not  hith- 
erto attempted  to  discriminate  the  effects  produced  by  the  dif- 
ferent constituents  of  ordinary  light,  the  thermal,  luminous, 
and  actinic  rays.  I  find,  however,  that  the  adoption  of  the 
word  "photophone"  by  Mr.  Tainter  and  myself  has  led  to  the 
assumption  that  we  believed  the  audible  effects  discovered  by 
us  to  be  due  entirely  to  the  action  of  luminous  rays.  The 
meaning  we  have  uniformly  attached  to  the  words  u  photo- 
phone"  and  "light"  will  be  obvious  from  the  following  pas- 
sage, quoted  from  my  Boston  paper: 

"  Although  effects  are  produced  as  above  shown  by  forms  of 
"  radiant  energy,  which  are  invisible,  we  have  named  the  appa- 
"  ratus  for  the  production  and  reproduction  of  sound  in  this  way 
"  the  '  photophone '  because  an  ordinary  beam  of  light  contains 
"  the  rays  which  are  operative" 

To  avoid  in  future  any  misunderstandings  upon  this  point  we 
have  decided  to  adopt  the  term  "  radiophone"  proposed  by  M. 
Mercadier,  as  a  general  term  signifying  an  apparatus  for  the 
production  of  sound  by  any  form  of  radiant  energy,  limiting 
the  words  thermophone,  photophone^  and  actinophone  to  appa- 


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37 

ratus  for  the  production  of  sound  by  thermal,  luminous,  or 
actinic  rays  respectively. 

M.  Mercadier,  in  the  course  of  his  researches  in  radiophony, 
passed  an  intermittent  beam  from  an  electric  lamp  through  a 
prism,  and  then  examined  the  audible  effects  produced  in  dif- 
ferent parts  of  the  spectrum.  (Co'mptes  Rendus,  Dec.  6th,  1 880.) 

We  have  repeated  this  experiment,  using  the  sun  as  our  source 
of  radiation,  and  have  obtained  results  somewhat  different  from 
those  noted  by  M.  Mercadier. 

(1.)  A  beam  of  sunlight  was  reflected  from  a  heliostat  (A, 
Fig.  12)  through  an  achromatic  lens,  (B,)  so  as  to  form  an 
image  of  the  sun  upon  the  slit  (C.) 

The  beam  then  passed  through  another  achromatic  lens  (D) 
and  through  a  bisulphide  of  carbon  prism,  (E,)  forming  a  spec- 
trum of  great  intensity,  which,  when  focused  upon  a  screen, 
was  found  to  be  sufficiently  pure  to  show  the  principal  absorp- 
tion lines  of  the  solar  spectrum. 

The  disk-interrupter  (F)  was  then  turned  with  sufficient  ra- 
pidity to  produce  from  five  to  six  hundred  interruptions  of  the 
light  per  second,  and  the  spectrum  was  explored  with  the  re- 
ceiver, (G,)  which  was  so  arranged  that  the  lamp-black  surface 
exposed  was  limited  by  a  slit,  as  shown. 

Under  these  circumstances  sounds  were  obtained  in  every 
part  of  the  visible  spectrum,  (excepting  the  extreme  half  of  the 
violet,)  as  well  as  in  the  ultra-red.  A  continuous  increase  in 
the  loudness  of  the  sound  was  observed  upon  moving  the  re- 
ceiver (G)  gradually  from  the  violet  into  the  ultra-red.  The 
point  of  maximum  sound  lay  very  far  out  in  the  ultra-red.  Be- 
yond this  point  the  sound  began  to  decrease,  and  then  stopped 
so  suddenly  that  a  very  slight  motion  of  the  receiver  (G)  made 
all  the  difference  between  almost  maximum  sound  and  complete 

silence.* 

(2.)  The  lamp-blacked  wire  gauze  was  then  removed  and  the 
interior  of  the  receiver  (G)  was  filled  with  red  worsted.  Upon 
exploring  the  spectrum  as  before,  entirely  different  results  were 
obtained.  The  maximum  effect  was  produced  in  the  green  at 

*  The  results  obtained  in  this  and  subsequent  experiments  are  shown  in  a 
tabulated  form  in  Fig.  14. 


38 

that  part  where  the  red  worsted  appeared  to  he  black.  On 
either  side  of  this  point  the  sound  gradually  died  away,  becom- 
ing inaudible  on  the  one  side  in  the  middle  of  the  indigo,  and 
on  the  other  at  a  short  distance  outside  the  edge  of  the  red. 

(3.)  Upon  substituting  green  silk  for  red  worsted  the  limits 
of  audition  appeared  to  be  the  middle  of  the  blue  and  a  point 
a  short  distance  out  in  the  ultra-red.  Maximum  in  the  red. 

(4.)  Some  hard-rubber  shavings  were  now  placed  in  the  re- 
ceiver (G.)  The  limits  of  audibility  appeared  to  be  on  the  one 
hand  the  junction  of  the  green  and  blue,  and  on  the  other  the 
outside  edge  of  the  red.  Maximum  in  the  yellow.  Mr.  Tainter 
thought  he  could  hear  a  little  way  into  the  ultra-red,  and  to 
his  ear  the  maximum  was  about  the  junction  of  the  red  and 
orange.* 

(5.)  A  test-tube  containing  the  vapor  of  sulphuric  ether  was 
then  substituted  for  the  receiver  (G.)  Commencing  at  the 
violet  end,  the  test  tube  was  gradually  moved  down  the  spec- 
trum and  out  into  the  ultra-red  without  audible  effect,  but 
when  a  certain  point  far  out  in  the  ultra-red  was  reached  a  dis- 
tinct musical  tone  suddenly  made  its  appearance,  which  disap- 
peared as  suddenly  on  moving  the  test-tube  a  very  little 
further  on. 

(6.)  Upon  exploring  the  spectrum  with  a  test-tube  contain- 
ing the  vapor  of  iodine  the  limits  of  audibility  appeared  to  be 
the  middle  of  the  red  and  the  junction  of  the  blue  and  indigo. 
Maximum  in  the  green. 

(7.)  A  test-tube  containing  peroxide  of  nitrogen  was  substi- 
tuted for  that  containing  iodine.  Distinct  sounds  were  ob- 
tained in  all  parts  of  the  visible  spectrum,  but  no  sounds  were 
observed  in  the  ultra-red. 

The  sounds  were  well  marked  in  all  parts  of  the  violet,  and 
I  even  fancied  that  the  audible  effect  extended  a  little  way  into 
the  ultra-violet,  but  of  this  I  cannot  be  certain.  Upon  exam- 
ining the  absorption  spectrum  of  peroxide  of  nitrogen  it  was  at 
once  observed  that  the  maximum  sound  was  produced  in  that 
part  of  the  spectrum  where  the  greatest  number  of  absorption 
lines  made  their  appearance. 

*  Iu  the  diagram  Fig.  14  the  mean  of  these  readings  is  shown. 


41 

(8.)  The  spectrum  was  now  explored  by  a  selenium  cell,  and 
the  audible  effects  were  observed  by  means  of  a  telephone  in 
the  same  galvanic  circuit  with  the  cell.  The  maximum  effect 
was  produced  in  the  red  about  its  junction  with  the  orange. 
The  audible  effect  extended  a  little  way  into  the'  ultra-red  on 
the  one  hand  and  up  as  high  as  the  middle  of  the  violet  on  the 
other. 

Although  the  experiments  so  far  made  can  only  be  considered 
as  preliminary  to  others  of  a  more  refined  nature,  I  think  we 
are  warranted  in  concluding  that  the  nature  of  the  rays  that 
produce  sonorous  effects  in  different  substances  depends  upon 
the  nature  of  the  substances  that  are  exposed  to  the  beam,  and 
that  the  sounds  are  in  every  case  due  to  those  rays  of  the  spec- 
tram  that  are  absorbed  by  the  body. 

The  Spectrophone. 

Our  experiments  upon  the  range  of  audibility  of  different 
substances  in  the  spectrum  have  led  us  to  the  construction  of  a 
new  instrument  for  use  in  spectrum  analysis,  which  was  de- 
scribed and  exhibited  to  the  Philosophical  Society  of  Washing- 
ton last  Saturday.*  The  eye-piece  of  a  spectroscope  is  re- 
moved, and  sensitive  substances  are  placed  in  the  focal  point  of 
the  instrument  behind  an  opaque  diaphragm  containing  a  slit. 
These  substances  are  put  in  communication  with  the  ear  by 
means  of  a  hearing-tube,  and  thus  the  instrument  is  converted 
into  a  veritable  "  spectrophone,"  like  that  shown  in  Fig.  13. 

Suppose  we  smoke  the  interior  of  our  spectrophonic  receiver, 
and  fill  the  cavity  with  peroxide  of  nitrogen  gas.  We  have 
then  a  combination  that  gives  us  good  sounds  in  all  parts  of 
the  spectrum,  (visible  and  invisible,)  except  the  ultra  violet. 
Now,  pass  a  rapidly-interrupted  beam  of  light  through  some 
substance  whose  absorption  spectrum  is  to  be  investigated,  and 
bands  of  sound  and  silence  are  observed  upon  exploring  the 
spectrum,  the  silent  positions  corresponding  to  the  absorption 
bands.  Of  course,  the  ear  cannot  for  one  moment  compete 
with  the  eye  in  the  examination  of  the  visible  part  of  the  spec- 

*Proc.  of  Phil.  Soc.  of  Washington,  April  16,  1881. 


42 

trum  ;  but  in  the  invisible  part  beyond  tbe  red,  where  the  eye 
is  useless,  the  ear  is  invaluable.  In  working  in  this  region  of 
the  spectrum,  lamp-black  alone  may  be  used  in  the  spectro- 
phonic  receiver.  Indeed,  the  sounds  produced  by  this  sub- 
stance in  the  ultra-red  are  so  well  marked  as  to  constitute  our 
instrument  a  most  reliable  and  convenient  substitute  for  the 
therrno-pile.  A  few  experiments  that  have  been  made  may  be 
interesting. 

(1.)  The  interrupted  beam  was  filtered  through  a  saturated 
solution  of  alum. 

Result :  The  range  of  audibility  in  the  ultra-red  was  slightly 
reduced  by  the  absorption  of  a  narrow  band  of  the  rays  of  lowest 
refrangibility.  The  sounds  in  the  visible  part  of  the  spectrum 
seemed  to  be  unaffected. 

(2.)  A  thin  sheet  of  hard  rubber  was  interposed  in  the  path 
of  the  beam. 

Result :  Well-marked  sounds  in  every  part  of  the  ultra-red. 
No  sounds  in  the  visible  part  of  the  spectrum,  excepting  the 
extreme  half  of  the  red. 

These  experiments  reveal  the  cause  of  the  curious  fact  al- 
luded to  in  my  paper  read  before  the  American  Association 
last  August — that  sounds  were  heard  from  selenium  when  the 
beam  was  filtered  through  both  hard  rubber  and  alum  at  the 
same  time.  (See  table  of  results  in  Fig.  14.) 

(3.)  A  solution  of  ammonia-sulphate  of  copper  was  tried. 

Result :  When  placed  in  the  path  of  the  beam  the  spectrum 
disappeared,  with  the  exception  of  the  blue  and  violet  end.  To 
the  eye  the  spectrum  was  thus  reduced  to  a  single  broad  band 
of  blue-violet  light.  To  the  ear,  however,  the  spectrum  re- 
vealed itself  as  two  bands  of  sound  wTith  a  broad  space  of  silence 
between.  The  invisible  rays  transmitted  constituted  a  narrow 
band  just  outside  the  red. 

I  think  I  have  said  enough  to  convince  you  of  the  value  of 
this  new  method  of  examination,  but  1  do  not  wish  you  to 
understand  that  we  look  upon  our  results  as  by  any  means 
complete.  It  is  often  more  interesting  to  observe  the  first  tot- 
terings  of  a  child  than  to  watch  the  firm  tread  of  a  full- 
grown  man,  and  I  feel  that  our  first  footsteps  in  this  new  field 


45 

of  science  may  have  more  of  interest  to  yon  than  the  fuller 
results  of  mature  research.  This  must  be  my  excuse  for  having 
dwelt  so  long  upon  the  details  of  incomplete  experiments. 

I  recognize  the  fact  that  the  spectrophone  must  ever  remain 
a  mere  adjunct  to  the  spectroscope,  but  I  anticipate  that  it  has 
a  wide  and  independent  iield  of  usefulness  in  the  investigation 
of  absorption  spectra  in  the  ultra-red. 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  OEPT. 

This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


Mis 


10 


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